Optimized fischer-tropsch catalyst

ABSTRACT

A cobalt containing catalyst supported on a metal oxide suitable for performing a Fischer-Tropsch reaction. A pore volume of a metal oxide support, before loading of cobalt thereon, is within the range of 0.35 to 0.85 cc/g. The support has an average pore diameter before the cobalt loading and reduction such that the effective average pore diameter after cobalt loading and reduction is 14 nanometers or higher. A cobalt loading of 11 weight % or higher is also provided. An alpha value higher than 0.89 in a diesel to wax weight ratio below 1.07 is provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. Conversion Utility patent application Ser. No. 14/546,132, filedNov. 18, 2014, which claims priority to U.S. Provisional PatentApplication Ser. No. 61/906,166, filed Nov. 19, 2013, entitled“OPTIMIZED FISCHER-TROPSCH CATALYST,” the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of heavy hydrocarbonproducts from light gaseous hydrocarbons such as natural gas, associatedgas, coal seam gas, landfill gas, or biogas. In particular, the presentinvention relates to optimization of a catalyst for performing aFischer-Tropsch reaction.

2. Related Art

Various processes are known for the conversion of carbonaceous feeds orlight hydrocarbons containing gases into normally liquid products suchas methanol, higher alcohols and hydrocarbon fuels and chemicalsparticularly paraffinic hydrocarbons. Such processes are directed at theobjective of adding value to the feedstock by making a transportable,more valuable product such as diesel fuel or jet fuel or chemicals suchas base oils or drilling fluids.

The Fischer-Tropsch process can be used to convert such feedstocks intomore valuable easily transportable liquid hydrocarbon products andchemicals. The feedstock is first converted to synthesis gas comprisingcarbon monoxide and hydrogen. The synthesis gas is then converted toheavy hydrocarbon products over a Fischer-Tropsch catalyst. The heavyhydrocarbon products can be subjected to further workup byhydroprocessing such as hydrocracking and/or hydroisomerization anddistillation resulting in, for example, a high yield of high qualitymiddle distillate products such as jet fuel or diesel fuel. The heavyhydrocarbon products can also be upgraded to specialty products such assolvents, drilling fluids, waxes, or lube base oils due to the highpurity of the Fischer-Tropsch products.

Processes that convert light hydrocarbons to heavier hydrocarbonproducts, for example, generally have three steps: 1) conversion oflight hydrocarbon feedstock to synthesis gas comprising carbon monoxideand hydrogen; 2) conversion of the synthesis gas to heavy hydrocarbonsvia the Fischer-Tropsch reaction; and 3) hydroprocessing the heavyhydrocarbon product to one or more finished hydrocarbon products.

The design and optimization of the Fischer-Tropsch reactor is ofparamount importance for the technical and economical success of a plantfor the conversion of synthesis gas into hydrocarbons. A Fixed BedFischer Tropsch (FBFT) reactor is a very simple effective reactor thatis very scalable.

A FBFT reactor must meet many conditions such as minimum complexity,ease of construction, minimum number of tubes, high selectivity towardsdesired products, high per pass conversion to avoid a second or a thirdstage, low pressure drop, etc.

It follows that the design of a FBFT reactor cannot be done withouttaking into account the characteristics and performance of theFischer-Tropsch catalyst to be used.

This is a technical challenge that involves many variables. FIG. 1 is anattempt to visualize some of the main variables involved and to clarifytheir interaction.

For example, while it is known that a high activity catalyst is desired,this has an effect on the operating temperature (“T”). If the operatingtemperature is too high, the Fischer-Tropsch reaction rate will be highas well, increasing the possibility of a temperature excursion or runaway. To avoid this, the reactor tube diameter (“D”) has to be smallerto facilitate the radial heat transfer. The overall heat transfer alsoincreases with the gas linear velocity (“LV”), but increasing the linearvelocity increases the pressure drop (“ΔP”) from the top to the bottomof the reactor.

To lower the pressure drop at any given set of conditions, the catalystparticle size has to increase, which may result in poor selectivitiesdue to diffusion considerations. An alternative would be to lower thetube height, although this could decrease the per pass conversion (“PPCO conv”) of the carbon monoxide and, therefore, either increase theinternal recycle or add another Fischer-Tropsch stage to reach thedesired total CO conversion. This could also result in a larger numberof shorter reactors therefore increasing the plant complexity and thecapital cost. While taking all these considerations into account, theselectivity towards the desired products must not decrease.

It is therefore apparent that while the reactor design has to be easy tofabricate, it has to take into account the catalyst performance. Thecatalyst has to be large enough to minimize the pressure drop and toallow for an optimal reactor height based on a targeted CO per passconversion, but not so large that it will cause a negative effect of thediffusion on the desired selectivity.

At the same time, the catalyst particle has to minimize the diffusioneffect for any given particle size. This can be achieved by having poresof a diameter large enough so that the product's selectivity is notaffected. On the other hand, a large pore diameter, associated with apore volume large enough to accommodate a selected amount ofFischer-Tropsch active metal such as cobalt without narrowing the porediameter too much and, therefore, cause diffusion problems, will weakenthe mechanical strength of the catalyst particle. A low catalystparticle mechanical strength will cause catalyst breakage during thecatalyst loading step and therefore increase the pressure drop to levelsabove design.

It is therefore necessary to develop a support for a Fischer-Tropschcatalyst that can meet all the above expectations in order to design atechnically and economically viable Fischer-Tropsch process.

The current state of the art for fixed bed Fischer-Tropsch catalystsdoes not address all these issues and the technology described in theopen art and in the patent literature is typically focused on a few ofthese considerations at a time, ignoring the negative effect thatoptimizing only one variable may have on the other variables.

Another problem in the art is the “open” optimization of the targetedparameters. That is, the optimized parameter ranges from a certain valueto an infinite or zero value. In those cases, following the teachingsmay lead to technical failures.

Because of the above considerations, it is necessary to develop aFischer-Tropsch Fixed Bed catalyst that can meet all the requirementsfor the construction of a technically and economically viableFischer-Tropsch reactor for the selective conversion of synthesis gasinto valuable hydrocarbon products. It is also desirable to develop aFischer-Tropsch slurry bubble column reactor catalyst that can meet allthe requirements for the construction of a technically and economicallyviable Fischer-Tropsch reactor for the selective conversion of synthesisgas into valuable hydrocarbon products.

SUMMARY OF THE INVENTION

The present invention deals with the development of an optimizedFischer-Tropsch catalyst that has enhanced selectivity towards desirableFischer-Tropsch products while at the same time having mechanicalstrength as well as pressure drop characteristics and intrinsic activitysuitable for optimal design and use in commercial Fischer-Tropsch FixedBed reactors.

The catalyst of the present invention has a defined pore volume, adefined average pore diameter, and a defined particle size.

While examples given and optimal support particle size discussion hereinare related to a fixed bed reactor, the catalyst of the presentinvention could also be provided in a powder form and be useful in aslurry bubble column reactor (“SBCR”). For a SBCR, the particle sizewill be defined by the available spray dried support material typicallyin the 20 to 250 μm range, not the optimum support size for diffusionconsiderations described herein for a fixed bed reactor of between 0.4and 1 mm.

In addition, it has been found that the crush strength of the extrudatescan be greatly enhanced by one or more treatments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart illustrating parameters for the design of an optimalFischer-Tropsch fixed bed reactor according to the present invention.

FIG. 2 is a graph illustrating catalyst activity versus metal loading.

FIG. 3 is a graph illustrating support particles crushing strengthversus pore volume.

FIG. 4 is a graph illustrating the total amount of active metalimpregnation versus support pore volume.

FIG. 5 is a graph illustrating methane selectivity versus averagesupport pore size.

FIG. 6A, FIG. 6B and FIG. 6C are graphs illustrating average supportpore diameter versus surface area.

FIG. 7 is a graph illustrating pressure drop versus catalyst particlediameter.

FIG. 8 is a graph illustrating experimental methane selectivity versuspredicted selectivity.

FIG. 9 is a graph illustrating experimental methane selectivity versuspredicted selectivity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments discussed herein are merely illustrative of specificmanners in which to make and use the invention and are not to beinterpreted as limiting the scope of the instant invention.

While the invention has been described with a certain degree ofparticularity, it is to be noted that many modifications may be made inthe details of the invention's construction and the arrangement of itscomponents without departing from the spirit and scope of thisdisclosure. It is understood that the invention is not limited to theembodiments set forth herein for purposes of exemplification.

A suitable Fischer-Tropsch catalyst has to have a high intrinsicactivity. Preferably, this high catalyst intrinsic activity can be usedin a fixed bed reactor in order to have a maximum productivity per unitvolume while at the same time meeting the desired heat transferrequirements.

If the intrinsic activity of the catalyst is high enough so that thepotential productivity within the regime of desired selectivities ismore than that allowed by the overall heat transfer coefficient, definedamong other variables by the gas linear velocity inside the reactor,then the operating temperature has to be lowered in order to maintainthe Fischer-Tropsch productivity or reaction rate within the range ofthermal stability of the reactor.

Lowering of the operating temperature for any given supported catalystat a given set of operating conditions will decrease the unwantedmethane selectivity as well as increase the selectivity towards valuablehigher hydrocarbons.

When the cobalt is deposited in the catalyst by any suitable technique,it is normally calcined to convert the deposited cobalt into a cobaltoxide phase, prior to its reduction into cobalt metal, typically using ahydrogen containing gas at temperatures between 400° F. to 750° F. Tofacilitate the reduction process, reduction promoters such as Pt, Ru,Cu, Ag, Pd, or Re in amounts ranging from 0.01 to about 3 wt % may beadded. The function of these promoters is to enhance the ease ofreduction of the supported cobalt oxide into its metal phase, which isthe active phase for the Fischer-Tropsch catalyst.

A second class of promoters is also used in the formulation of theFischer-Tropsch catalysts. This second type of promoter has a range ofdifferent objectives. A non-restrictive list of such objectives includeenhancing the mechanical strength of the support, its chemicalresistance towards phase changes of the support during theFischer-Tropsch reaction, to change the support acidity or basicity, tolower the support activity, to have a degree of control over the averagecobalt crystallite sizes and to modify the metal support interaction.Examples of second type of promoters are Mn, Ba, Si, Ti, Ce, Zr, La, Na,K, Zn, Ag, Cu, Li, Ca, Cr, Ni, Fe, V, Sn, Ga, Sb, Th, and W in amountsranging from 0.1 to 15 wt %.

Catalyst Production/Activity vs. Cobalt Loading

It is known by those familiar with the art that the catalyst activity isa strong function, among other variables, of the catalyst loading or theamount of active metal impregnated. Since the active metal for theFischer-Tropsch reaction is cobalt in its metallic form, then it followsthat the more cobalt per catalyst particle external surface area, thehigher the Fischer-Tropsch activity.

For discussion herein, the definition of the active metal loaded or Cowt % or cobalt impregnated or loaded is approximately 80% of the cobaltloaded as being in the metallic form and 20% of the cobalt loaded beingpresent as CoO after reduction. That is, the sum of both the CO metaland CoO are used to calculate the wt % of the cobalt loaded.

To ascertain the effect of catalyst loading on the Fischer-Tropschcatalyst intrinsic activity, six catalysts were prepared and tested in afixed bed micro reactor with an ID of 12 mm, a length of 40 cm, andloaded with Fischer-Tropsch catalyst diluted in crushed particles ofsilicon carbide of roughly the same size as the catalyst particles witha ratio of diluents to catalyst of 6:1 by volume.

The catalysts were prepared by multiple impregnations of a mixture ofcobalt nitrate in water on an alumina support, with calcinations in airin between impregnations to convert the cobalt nitrate to cobalt oxidebefore the next impregnation.

The range of operating pressures was from approximately 390 to 410 psigand the range of the operating temperature was from approximately 380°F. to 420° F. The activity was calculated after about 120 hours on lineby means of the Huff-Satterfield kinetic equation, shown below asEquation (1):

FT_(r) =−r _(CO) =K _(t) e ^((ΔE/RT)) P _(H2) P _(CO)/(1+a P _(CO))²  (1)

Where,

a=x1+x2×T(° K)   (2)

-   FT_(r)=−r_(CO) is the Fischer-Tropsch rate expressed in cc CO    reacted per cc catalyst per hour.-   P_(CO) and P_(H2) are the kinetic average partial pressures of CO    and H₂, in atm-   T is the reaction temperature, in ° K-   ΔE is the activation energy-   R is the Universal gas constant.-   ΔE/R=−8000-   “a” is the desorption coefficient, and-   K_(t) is the pre-exponential factor or catalyst intrinsic activity    at time “t”

The intrinsic activity of the catalyst is the activity to perform theFischer-Tropsch reaction independently of the operating conditions underwhich the reaction is taking place.

By using Equation (1), all the 6 test runs can be compared equally,irrespective of differences in the operating conditions.

TABLE 1 Operating Cat Co Conditions FT Test No No Wt % P (psig) T (F.)H2/CO rate Kt × 10⁻¹⁰ 1 A084 20 410 415 2.0 408 1.68 2 A087 20 400 4152.0 380 1.70 3 A090 20 410 420 2.0 375 1.59 4 A097 30 410 404 2.0 6602.75 5 A098 30 410 392 1.8 590 3.03 6 A229 40 390 380 2.0 444 4.13

The results from Tests 1 through 6 are shown in Table 1 and are showngraphically in FIG. 2. Table 1 shows each of the tests with the cobaltweight by percentage and the various operating conditions. From theresults, it is evident that the higher the loading of the catalystactive metal, the higher the Fischer-Tropsch activity. It is alsoevident that at some point, the loading of too much active metal mayresult in the narrowing of the pore diameter with a resulting diffusionrelated higher methane selectivity and it may even reach regimes wherethe activity actually decreases, as more metal is added.

This was not the case for the Tests 1 through 6, where the activityversus metal loading showed an almost linear behavior. By looking onlyat these examples, it seems clear it is desirable to have as much activemetal as possible on the Fischer-Tropsch catalyst. In order to avoid thenegative effect of diffusion as the metal loading increases, the porevolume should increase correspondingly.

Catalyst Commercial Preparation—Effect of Pore Volume

When preparing an aqueous impregnation of cobalt nitrate on a metaloxide support, because of the limited solubility of cobalt nitrate inwater, there is a maximum amount of equivalent active metal that can bedeposited per impregnation step. This maximum amount is also a functionof the support pore volume.

The higher the pore volume, the higher the amount of active metal thatcan be deposited per impregnation step.

This is shown in FIG. 4, where the total amount of active metal perimpregnation has been calculated as a function of the pore volume. Thereduction of the pore volume after each subsequent impregnation has beentaken into account.

In a commercial preparation for an impregnated Fischer-Tropsch catalyst,each impregnation is followed by drying and calcinations in rotary kilnsbefore the material is ready for the next impregnation. This means thatthe higher the number of impregnations to reach a targeted active metalloading, the more expensive and complicated the catalyst commercialmanufacturing process is.

From FIG. 4, and from a commercial catalyst preparation consideration,it can be observed that the catalyst pore volume should be as high aspossible in order to achieve high cobalt loading which is desirable insome commercial applications.

To determine the minimum amount of cobalt loading necessary for apractical commercial application, a simulation of different cobaltloadings and its effect on the reactor performance is described inExample 1.

EXAMPLE 1

As an example of the effect of the active metal loading or catalystintrinsic activity on the operating temperature of a commercialFischer-Tropsch Fixed Bed reactor, the loading of 5 different catalystswas simulated, 10, 15 20, 30 and 40 cobalt wt % respectively, using aplug flow reactor model.

This model consists of 100 increments along the reactor. After eachincrement, the new partial pressures, temperature, and gas linearvelocities are calculated. The rate of reaction was calculated usingEquation (1).

The desired gas inlet linear velocity, calculated on an empty tube, was24 cm/s, the catalyst bed height in this example is 18 feet, and thepressure drop is 25 psi. The reactor was running with a recycle ratio(mols recycle/mols feed) of 0.7 and the overall CO desired conversion is90% .

The feed composition consists of 59.64 vol % H2, 28.92 vol % CO, 1.46vol % CH4, 6.69 vol % CO2, and the balance is N2 and Ar.

The calculated average reactor temperatures to achieve the desired totalCO conversion of 90% and the resulting methane selectivities (usingEquation (9) to be described later on) for the different active metalloadings or cobalt wt % are:

Co Wt % 10 15 20 30 40 Temperature (° F.) 434 419 407 386 376 CH4selectivity, C wt % 12.8 11.3 10.1 8.6 7.8

In a Fischer-Tropsch commercial plant, it is desirable that the methaneselectivity be below 12 wt %, preferably below 10 wt %. In general, asthe methane selectivity increases, the overall efficiency of the plantdecreases. If the plant uses natural gas as feed, then it is verynegative from an economic point of view to transform methane intosynthesis gas and then to react the synthesis gas in a Fischer-Tropschreactor and produce methane that is the plant original feed.

A cobalt loading of 10 wt % will have a low activity, making itnecessary to increase the operating temperature to 434° F., obtaining amethane selectivity of 12.8 C wt %. Although the catalyst with cobaltloading of 15 wt % has a methane selectivity below 12 wt %, thereforesuitable for commercial application, a catalyst with at least a 20 wt %cobalt loading is preferred since its methane selectivity is practically10 C wt %

FIG. 4 also shows that after each impregnation, there are decreasingincrements in the total amount of active metal loaded. For about 20 wt %of active metal and using 4 impregnation steps, the pore volume shouldbe higher than about 0.35 cc/g. A higher volume will decrease the numberof impregnation steps.

Effect of High Pore Volume on Catalyst Mechanical Strength

Support extrudates were prepared using commercially scalable extrudersat different conditions to ascertain the effect of the preparationconditions on the support pore volume and on its mechanical strength.

The mechanical strength of the support is very important to avoidparticle breakage mainly during the impregnation and calcination steps,as well as during the transport of the catalyst from the manufacturingplant to the Fischer-Tropsch plant and the loading of the catalyst intothe Fischer-Tropsch reactor.

If there is particle breakage, the pressure drop during normalFischer-Tropsch operation will be higher than designed and, dependingupon the extent of the breakage, the catalyst may not even be suitablefor loading or, if the breakage occurred during loading, for operation.The minimum typical catalyst crush strength for FTFB reactors is about 1lb. per mm of extrudate.

Because of the above considerations, a program was developed to identifythe best set of conditions for the preparation of the catalyst support,as well as to identify the catalyst property/properties that influencethe mechanical strength of the support.

The preparation of the different alumina supports studied are asfollows:

Prior to the water addition, the alumina support is calcined at atemperature in excess of 1000° C. in order to measure the loss onignition (“LOI”). This preliminary step is important in order tocalculate the amount of water to be added to the uncalcined aluminapowder in order to obtain the desired LOI for extrusion. For thepurposes of this study, the targeted LOI range was from about 46% toabout 51%. In some cases, nitric or citric acid were added as apeptizing agent.

The alumina powder was placed in a container which could be a mixer or amuller. The liquid solution (water or water plus an acid) was added andthe mixture was mixed or mulled for 10 to 40 minutes. After this, thepaste was transferred to an extruder with 0.9 mm tri-lobe dies obtainedfrom JMP Laboratories Inc.

After extrusion, the extrudates were dried for about one hour at 140° C.followed by calcinations for one hour at temperatures from 900° C. to975° C. The calcined extrudates were tested for surface area, porevolume, pore diameter, and individual particle crush strength.

The preparation conditions and the properties of the calcinedextrudates, ready for impregnation, are shown in Table 2.

TABLE 2 Sample # 1 2 3 4 5 6 7 8 Calc T, ° C. 900 900 975 975 975 975950 975 LOI 47.1 47 46.9 47.5 50 50.7 47.3 50 Wt % Nitric 0.0008 0 0 0 00 0.0008 0 Wt % citric 0 0 0 0 0 2 0 0 Mull time, min 20 20 30 10 10 2010 20 SA, m2/g 83 81 76 74 68 68 80 79 MPV, mL/g 0.589 0.595 0.575 0.5880.603 0.568 0.543 0.531 MPD, nm 22.5 23.3 23.1 22.7 26 25.7 21.2 22.3Crush strength, 1.05 0.93 0.96 1.14 0.96 1.48 2.07 2.14 lb/mm

An analysis of the data showed that the most important parameter topredict the mechanical or crush strength of the extrudates is the porevolume. This is shown in FIG. 3, where the crush strength decreases asthe pore volume increases.

The crush strength limitation is a practical limit related to handlingof the catalyst during manufacturing. It has been found that as metalsare loaded onto the support, the crush strength of the finished catalystis higher than the support alone. However, there needs to be a minimallevel of strength for handling the support during manufacturing and 1lb./mm was arbitrarily selected as a minimum target.

It has now been found that the crush strength of the extrudates can begreatly enhanced by one or more treatments. For example, alumina can bestrengthened by addition of aluminum nitrate or by addition of a strongacid such as acetic acid or nitric acid. Acid can be added to thealumina before or after extrusion, but preferably is added beforeextrusion. After drying and calcining, the resulting alumina is muchstronger.

FIG. 3 shows two graph lines that relate crush strength to pore volume.The lower graph line is based on 0.9 mm extrudates made with aluminapowder that is mixed with water and in some cases a very low level ofacid. The line shows a general relationship of pore volume with crushstrength and demonstrates that as pore volume increases crush strengthdecreases. The relationship is very nearly linear.

The upper graph line of FIG. 3 is based on 0.9 mm extrudates made withthe same alumina however; approximately 12% by weight of concentrated(70%) nitric acid was added to the alumina powder before mixing,extrusion and calcining The treatment with a strong acid for thisparticular alumina obviously adds significant strength to the extrudatesmaking it possible to extrude alumina with much higher pore volume.Extending the line until crush strength drops to the minimum requirementof 1 lb./mm indicates that the pore volume could be as high as 0.85cc/gm. In fact with higher levels of acid, the pore volume couldpossibly be extended to even higher levels.

There are many ways to increase the pore volume of the alumina includingaddition of burnout agents and modification of the alumina crystalshape. Forming pellets or extrudates with such modified aluminas makesit possible to stay within desirable physical limits such as crushstrength while using alumina with much higher pore volume, making iteasier to reach desired metal loading targets with fewer impregnationsteps.

Effect of Pore Diameter on Methane Selectivity

Because of diffusion considerations, the pore diameter must not be toosmall if a high methane selectivity is to be avoided. The effect of poresize on diffusion for FT catalysts is discussed in a paper by E.Iglesia, et al, published on Advances in Catalysis, Volume 39, pages 221to 302. The methane selectivity is defined as the percentage of thecarbon monoxide reacted that is converted to methane.

The open and patent literature discuss this issue, typically referringto the pore diameter of the support, but not including the narrowingeffect of the loaded cobalt on the average pore diameter as well asrelating it with the necessary amount of cobalt to be loaded for apractical commercial application.

Because of this narrowing effect, it is more accurate to use the realpore diameter after the active metal has been loaded, calcined andreduced in order to ascertain the effect of pore diameter on thecatalyst performance.

In this invention, an effective average pore diameter is described asthe average pore diameter after loading of the FT active metalprecursor, calcinations to obtain the metal oxide, and reduction toobtain the active metal. For the calculations, we assumed a typicaltotal metal precursor reduction of 80%. In the case of cobalt, 80% ofthe cobalt loaded is reduced to cobalt metal and about 20% remains asCoO. This is, therefore, the degree of reduction that is part of thedefinition of the effective pore diameter. The densities used for thiscalculation were 8.9 g/cc for cobalt metal and 6.11 g/cc for CoO.

While it is understood that this is the typical degree of reduction, inpractice, the degree of reduction in a commercial Fischer-Tropschreactor may have some degree of variation, although the effect will beminimal for the purpose of these calculations.

The effective pore diameter is determined by calculating the volume ofthe impregnated active metal after the reduction of the cobalt oxide andsubtracting it from the original pore volume and recalculating the newpore diameter using the new pore volume and the initial surface area. Acalculation example follows.

EXAMPLE 2 Calculation of the Effective Pore Diameter

For the same family of metal oxide supports, e.g. silica, alumina,titania, etc., the average pore diameter, the total pore volume, and thesurface area are interrelated. Because of geometric considerations,changing two of these three variables will inevitably result in adifferent value for the third one. A typical equation used to expressthis relationship is shown in Equation 3 below:

Avg Pore diameter (nm)=4000×Pore volume (cc/g)/Surface area (m²/g)   (3)

Amount of reduced catalyst=100 g

Cobalt loading=40 Wt % or 40 g

Pre-impregnated support area and avg pore diameter=100 m2/g and 15 nm

Using Equation 3, the pore volume of the pre-impregnated support is 0.38cc/g

Total pore volume for 60 g support=60 g×0.38 cc/g=22.5 cc

Amount cobalt metal, Co :40 g ×80% reduction=32 grams

Amount cobalt oxide, CoO=40 g−32 g=8 g

Density Co and CoO=8.9 and 6.4 g/cc

Total volume Co and CoO=32 g/8.9 g/cc+8 g/6.4 g/cc=4.85 cc

Total pore vol after impregnations, calcinations andreduction=22.5−4.85=17.65 cc

New pore volume=17.65 cc/60 g support=0.29 cc/g

Assuming the same initial area and using equation (3), the new averagepore size diameter or effective average pore diameter is 11.7 nm.

The use of this technique to calculate the effective pore diameter is asimplification but it is believed to be better than just calculating thenew pore diameter by distributing the cobalt volume evenly over thecatalyst surface. This, because the support is formed by anagglomeration of very small particles, and the impregnated cobalt isdeposited in between them with a corresponding enhanced effect on thenarrowing of the pores (or open volume in between particles).

This approximation is believed to be more accurate than to merely usethe support pore diameter without taking into account the narrowing ofthe pores effect due to the amount of active metal to be loaded.

An example of the effect of the effective pore diameter on the methaneselectivity is shown in the following example. The particle size of thecatalysts tested in Table 3 was between 0.4 to 0.55 mm.

TABLE 3 Avg. pore Effective pore CH4 Selectivity Test No. Cat. No. size,nm size, nm (C wt %) 1 A084 8.1 7.7 11.8 2 A087 16.6 15.7 10.5 3 A09015.1 14.2 11.7 4 A097 8.1 7.4 15.5 5 A098 8.1 7.4 13.0 6 A229 17.5 14.98.5The results from Table 3 are plotted in FIG. 5. As previously discussed,in a commercial plant, it is desirable to have a methane selectivitybelow 12 C wt %, preferably below 10 C wt %. To achieve this, theeffective pore diameter must be greater than about 14 nm.

The effective pore diameter of greater than 14 nm must also be expressedtaking into account both the surface area and the upper and lower limitsfor the total pore volume, defined by the mechanical strength of thesupport and by the ease of commercial manufacture of the catalyst,respectively.

For example, if a support of area 100 m²/g is loaded with 40 wt % of theactive metal, in order to have an effective pore diameter of at least 14nm, the support must have an average pore diameter of 17.2 nm and a porevolume of 0.43 cc/g.

If a support has an area of 150 m²/g and is loaded with the same amountof active metal, in order to have an effective pore diameter of at least14 nm, it must have an initial average pore diameter of 16.2 nm and apore volume of 0.61 cc/g.

If the support has an area of 70 m²/g and loaded with the same amount ofactive metal, in order to have an effective pore diameter of at least 14nm, it must have an initial average pore diameter of 18.6 nm and a porevolume of 0.3 cc/g. Here, the pore volume is outside the catalystcommercial manufacture requirements as the pore volume is too low.

Since the pore volume, pore diameter, and surface area are interrelated,as shown by Equation (3), it is possible to derive a general equationthat calculates the minimum average pore diameter of the support so thatafter impregnations, calcinations, and reduction of the active metal orcobalt, the effective pore diameter is at least 14 nm.

Because the extent of narrowing of the pore is a function of the amountof active metal loaded, this general equation will apply only for aspecific amount of cobalt loaded.

For each specific amount of cobalt loaded, e.g., 20, 30 and 40 wt %cobalt, a set of calculations were performed, each one for a differentsupport surface area. The purpose of these calculations was to determinethe initial support average pore diameter (before the cobalt loading andreduction) so that, for example, for a support of 100 m2/g and 30 wt %cobalt loaded, the effective average pore diameter will be 14 nm.

The results obtained are shown in FIG. 6 a. This Figure shows theaverage pore size diameter before impregnation that the support musthave so that, after the cobalt loading and reduction, the effectiveaverage pore diameter is 14 nm. As expected, each specific amount ofcobalt loading has its own curve. A statistical regression using a powertype equation (solid lines in FIG. 6 a was not very efficient to predictthe data.

Further analysis of the data gave a maximum correlation factor (r²) of0.997 when a constant (in this case 32 m2/g) was subtracted from thesurface area. The results are shown in FIG. 6 b.

The equations shown in FIG. 6 b as a function of the wt % of cobaltloading are shown in Equations (4a), (4b), (4c) and (4d).

Cobalt loading Equation 15 wt % Support avg pore diam = 17.485(Area-32)^(−0.038) (4a) 20 wt % Support avg pore diam = 19.012(Area-32)^(−0.052) (4b) 30 wt % Support avg pore diam = 22.9(Area-32)^(−0.083) (4c) 40 wt % Support avg pore diam = 28.427(Area-32)^(−0.118) (4d)The above equations are of the form:

Y=a X^(b)   (5)

It is possible to generalize Equations (4a) to (4d) by describing “a”and “b” in Equation (5) as a function of the cobalt loading.

A statistical regression was performed and the results are shown inEquations (6a) and (6b):

a=0.4372×wt % cobalt+10.48   (6a)

b=0.0113−0.0032×wt % cobalt   (6b)

A parity plot to test the accuracy of equations (6a) and (6b) showedthat the equations are very accurate, with correlation factors (r²) of0.987 and 0.998 respectively.

By combining Equations (4a to 4d) with Equations (6a) and (6b), it ispossible to arrive to a general equation that defines the average porediameter that the support must have, before cobalt loading andreduction, in order to have an effective pore size diameter of 14 nm.This general equation is shown as Equation (7).

Support APD=(0.4372×wt % Co+10.48)×(Area−32)^((0.0113−0.0032×wt % Co))  (7)

Where the average pore diameter or APD is expressed in nm and thesurface area in m²/g.

A comparison between the prediction of Equation (7) versus thecalculation procedure shown in Example 2 for the calculation of theeffective average pore diameter was performed for the range of 15 to 40wt % of cobalt and areas from 40 to 200 m2/g. The accuracy of Equation(7) was +/−2%.

The requirement for the effective average pore diameter for an optimumFischer-Tropsch catalyst, as previously discussed, is that it must be atleast 14 nm. This condition is now reflected in Equation (8).

Support APD=or >(0.4372×wt % Co+10.48)×(Area−32)⁽0.0113−0.0032×wt % Co)  (8)

From the above discussions, the optimum characteristics for the metaloxide support, for example alumina, for a Fischer-Tropsch cobaltcatalyst must be such that the conditions in Equation (8) must be metwithin a pore volume range of between 0.35 to 0.85 cc/g.

When taking into account that the support pore volume must be between0.35 cc/g to 0.85 cc/g, then it is possible to delineate a region whereall the requirements of pore volume (for mechanical strength andmanufacturing considerations) and minimum effective pore diameter aremet, that is, an optimal region. An example of this optimal region, fora cobalt loading of 30 wt %, is shown in FIG. 6 c as a shaded area.

The lower limit of the shaded area is defined by Equation (4c) (althoughEquation (7) would produce the same curve).

The limit on the right side of the shaded area is defined by a curve fora pore volume of 0.85 cc/g. The curve has been constructed by replacingthe “Pore Volume” in Equation (3) by 0.85. Any average support porediameter before impregnation and reduction above this curve will have apore volume greater than 0.85 cc/g and is outside the acceptablecharacteristics of an optimum support.

The limit on the left side of the shaded area is defined by a curve fora pore volume of 0.35 cc/g. The curve has been constructed by replacingthe “Pore Volume” in Equation (3) by 0.35. Any average support porediameter before impregnation and reduction below this curve will have apore volume lower than 0.35 cc/g and is outside the acceptablecharacteristics of an optimum support.

The upper limit of 40 nm in FIG. 6 c is defined by the effect of theaverage pore size on the cobalt crystallite size and, therefore, on thedispersion of the cobalt metal and its effect on the catalyst activity.

When the cobalt is loaded on a metal oxide support and reduced, itsphysical form is in spherically shaped crystallites. Typically, thecrystallite size is present in different sizes, ranging from about 2 toabout 35 nm or higher. It is known that the larger the average poresize, the larger the average crystallite size (e.g. U.S. Pat. No.7,541,310, FIG. 4). The crystallite cannot be larger than the specific“pore” or space in between support aggregates inside which it grows. Asupport with an average pore diameter of 40 nm may result in cobaltcrystallites with an average size of 30 nm or higher, depending on theamount of cobalt loaded. An average crystallite size of 30 nm willresult in a cobalt dispersion of about 3%. This means that only 3% ofthe cobalt metal is present in the external surface area of thespherical crystallite and available to catalyze the Fischer-Tropschreaction, while the remaining 97% of the cobalt is inside the sphericalcrystallite, unable to contribute to the reaction.

While larger crystallites are easier to reduce and more resistant tore-oxidation, they result in a low dispersion and, therefore, lowercatalyst activity for the Fischer-Tropsch reaction. On the other hand,smaller crystallites, although they have a potentially larger percentagecobalt metal on the external surface of the spherically shapedcrystallite, are more difficult to reduce and tend to re-oxidize (andtherefore deactivate the catalyst) faster than the larger crystallites.

Because of the above considerations, an optimum average cobaltcrystallite size should be larger than about 5 nanometers and smallerthan about 30 nm.

These considerations are the basis for the upper limit of 40 nm for theaverage pore diameter of the support before cobalt impregnations andreduction shown in FIG. 6 c.

The same approach can be taken for other cobalt loadings to producefigures similar to FIG. 6 c.

All the shaded area that delineates the optimal average pore size of thesupport before cobalt loading and impregnation as a function of wt % ofcobalt and support surface area before impregnation and reduction, bymeans of Equation (8), the two equations derived from Equation (3) forpore volumes of 0.85 cc/g and 0.35 cc/g and an upper limit for theaverage pore diameter of the support of 40 nm.

Effect of Extrudate Diameter

While the support can be an extrudate, tablet, oil drop sphere, crushedand sieved particle or any other shape known to one skilled in the art,extrudate is the preferred shape for our invention.

There are two main effects of the extrudate diameter besides its obviousinfluence on the mechanical strength for the optimization of aFischer-Tropsch catalyst for application in a fixed bed reactor: thepressure drop, which increases as the extrudate diameter decreases; andthe methane selectivity, which increases considerably above a criticalextrudate diameter.

A higher pressure drop inside the catalyst bed will also require ahigher inlet pressure to maintain a targeted average reaction pressure,therefore resulting in a larger gas compressor and higher operatingcosts. In the case of more than one Fischer-Tropsch stage, thecompression requirements become even more relevant and costly.

A high pressure drop will also require a higher mechanical strength ofthe catalyst particles, particularly if they are in the shape ofextrudates, to avoid their breakage, which in turn will further increasepressure drop.

Because of the above considerations, it is preferable to developcatalyst particles that will, because of their physical characteristics,cause a pressure drop as low as possible under any given set ofoperating conditions.

FIG. 7 shows the expected pressure drop in psi/ft for a fixed bedreactor running at typical commercial conditions. For thesecalculations, the Ergun equation was used below, and a syngas feed withan H₂/CO ratio of 2:1, 10% inerts (e.g. nitrogen), 350 psig inletreactor pressure, a CO per pass conversion of 60%, and an alpha value of0.92. The particle size ratio of Length/Diameter used was 2:1.

Notice that the pressure drop shows an exponential type of response withthe catalyst particle size, being particularly pronounced for particlesizes below 0.4 mm.

Because of this behavior, and due to pressure drop considerations, it ispreferred that the extrudate have a diameter larger than 0.4 mm.

The effect of the catalyst particle size on the undesirable methaneselectivity is shown in Table 4. In this example, all the catalystssupports were prepared using the same batch of alumina to avoid thepossible effect of the metal oxide support on the methane selectivity.They all have similar effective pore diameter that are within the targetlimits described by FIG. 6 c.

TABLE 4 Effect of the particle size on the methane selectivity DiameterTemp Press CH4 sel Catalyst μm ° F. psig H2/CO in CO Conv % Wt % A00841000 Extrudate 410 420 1.9-2 30-35 27 CO133-1 800 shaped extrudate 394405 1.94 56 11.9 A0087 500-600 crushed 415 420 1.3-2 65-70 9-11  A00108500 extrudate 395 400 1.9 57-65 8-9.3 A00216 400 extrudate 380 400 2 305-7  Table 4 shows that, in spite of optimal support characteristics asdescribed by the present invention, methane selectivity is stillstrongly influenced by the particle size. The methane selectivityincreases with the catalyst particle diameter, although the selectivityvalue shows a jump when the catalyst particle, in this case theextrudate diameter, reached a value of 1000 μm, or 1 mm.

This large increase in the methane selectivity is present even thoughthe range of operating conditions falls within the group range.Therefore, it is preferable that the catalyst particle size be below 1mm diameter.

To further ascertain the effect of the catalyst particle size, apredictive equation (Equation 9) for the methane selectivity wasdeveloped by means of varying the temperature and the partial pressureof reactants and products. The micro reactor runs also includeddifferent Fischer Tropsch reaction rates, expressed in ccCO reacted percc catalyst per hour and three different catalyst extrudate diameters:0.4, 0.6 and 0.8 mm.

This predictive methane selectivity equation was developed for catalystswith a support that falls within the optimal shaded region in FIG. 6 c.The range of operating conditions used for the development of Equation(9) is shown in Table 5.

TABLE 5 Range of operating conditions for the development of Equation(7) FT rate 128 1084 H2/CO ratio 1.27 2.04 PH2 2.1 11.3 PCO 1.6 5.8 PH2O0.4 2.9 t 100 3600 T 375 395 CH4 Sel 6.4 18.8

CH₄ Sel_(t−t)=3.48×10⁶ t ^(−0.06) e ^((ΔE/RT)) *P _(H2) *P _(CO) ^(−1.2)*P _(H2O) ^(−0.6)+CF   (9)

The CF term is the correction factor for the FT rate and the particlediameter, expressed as:

CF=((0.0312×PD)×FT rate−(6.264×PD−1.518))   (10),

Where:

CH4 Set=% Mols CO reacted converted to CH4

t=run time in hours

ΔE/R=−6000

T=Temperature in K

Pi's=Partial pressures in atm

PD=Catalyst extrudate diameter in mm

FT rate=cc CO reacted per cc of catalyst per hour

A parity plot for the experimental methane selectivity versus thepredicted selectivity using Equation (9) is shown in FIG. 8. From thisfigure, it is apparent that Equation (9) is very accurate for theprediction of the methane selectivity (C wt %) as a function of theoperating conditions, time on line, the catalyst extrudate diameter, andthe Fischer-Tropsch reaction rate. Notice that catalyst extrudates ofabout 0.8 mm diameter had a methane selectivity well below 10 C wt %.

The inclusion of the Fischer-Tropsch reaction rate in Equation (9) isdue to the effect of high reaction rates on the diffusional load of thecatalyst pores. It is not sufficient to determine the effect of theparticle diameter without taking into account the reaction rate. Forexample, if a catalyst of 1 mm diameter is operated under conditions ofvery low CO conversion or reaction rate, its effect on the selectivitieswill be either very small or negligible. This, however, would not bepractical from a commercial point of view.

If we choose to ignore the combined effects of the particle diameter andof the reaction rate as expressed in Equation (10), equation (9)becomes:

CH₄Sel_(t=t)=3.48×10⁶ t ^(−0.06) e ^((ΔE/RT)) *P _(H2) *P _(CO) ^(−1.2)*P _(H2O) ^(−0.6)   (11)

FIG. 9 shows the same parity plot as that in FIG. 8, but now, usingEquation (11). An examination of FIG. 8 clearly shows the failure ofEquation (11) for the prediction of the experimental methaneselectivity. This also confirms the strong effect of the increasingcatalyst particle size on increasing the methane selectivity, which isenhanced even more at high Fischer-Tropsch reaction rates.

Preferred Fischer-Tropsch Selectivity

The product's selectivity for the Fischer-Tropsch products can beconveniently defined by the chain growth probability of the growinghydrocarbon chain absorbed on the active metal. This chain growthprobability is commonly referred to as “alpha” value, or “α”

Alpha is a measure of the probability that an adsorbed growing chainwill either desorb and exit the catalyst pore without further growth orgrow by one more carbon number. Normally methane and, to a certainextent, C2 hydrocarbons do not follow this rule, but for the C3+hydrocarbons, alpha is independent of the carbon number. Therefore, bydefinition, alpha can be calculated using Equation (12):

Alpha=Mols C _(n+1) in products/Mols C _(n) in products   (12)

Where n>2.

Whether the Fischer-Tropsch plant is oriented towards fuels orchemicals, it is preferable to have the primary products fromFischer-Tropsch as heavy as possible. This is because if the plant iswax or lubes (made from the Fischer-Tropsch wax) oriented, then thehigher the alpha value the higher the selectivity of the plant towardsthose desired products. If the plant is fuel oriented, theFischer-Tropsch wax will be subject to a hydrocracking process toconvert it to naphtha and diesel. In this case, the higher the alphavalue the higher the plant total selectivity towards diesel and thelower the plant total selectivity towards naphtha.

The Fischer-Tropsch diesel has a cetane number in excess of 70, zerosulphur compounds, and practically zero aromatics. This makes it anexcellent blend stock with crude derived diesel. Because of theseproperties, diesel is the preferred fuel product from low temperatureFischer-Tropsch plants.

The Fischer-Tropsch naphtha, on the other hand, although it also haszero sulphur and has practically zero aromatics, is composed by about97% of linear hydrocarbons and, therefore, its octane value is very low.This means that naphtha does not have much value as fuel unless itundergoes severe hydrotreatments that lowers its yield substantially.

Table 6 shows the primary Fischer-Tropsch selectivities as well as thefinal plant selectivities after the wax was treated in the hydrocracker.It can be shown graphically that there is an inflexion point withdiminishing returns for the selectivity of wax or for the selectivity ofdiesel as the final plant product. This point with diminishing returnsis at about an alpha value of 0.89. At this point, the diesel to waxweight ratio is 1.07.

Because of the above reasons, an optimum Fischer-Tropsch catalyst shouldhave an alpha value of at least 0.89.

TABLE 6 Effect of the alpha value on the selectivity of theFischer-Tropsch primary products and on the plant final products Alphavalue Products 0.93 0.92 0.91 0.9 0.89 0.88 0.87 Fischer-Tropsch primaryproducts C2 to C4 3.49 4.46 5.52 6.67 7.90 9.19 10.55 Naphtha 10.1412.42 14.72 17.03 19.29 21.49 23.61 Diesel 24.27 27.46 30.08 32.14 33.6434.61 35.10 Wax 54.49 48.07 42.07 36.56 31.57 27.10 23.14 (C20+) Weight0.45 0.57 0.72 0.88 1.07 1.28 1.52 ratio diesel/wax Overall Plant finalproducts C2 to C4 5.5 6.2 7.1 8.1 9.2 10.4 11.62 Naphtha 22.1 23.1 24.125.3 26.5 27.8 29.10 Diesel 64.9 63.1 61.1 59.0 56.7 54.2 51.68 OverallC5+ Plant final products Naphtha 25.41 26.75 28.30 30.03 31.91 33.9136.02 Diesel 74.59 73.25 71.70 69.97 68.09 66.09 63.98

Example of the Performance of a Catalyst Prepared According to theTeachings of the Prevent Invention

A support with physical properties within the shaded area of FIG. 6 wasimpregnated multiple times with a solution of cobalt nitrate diluted inwater following the incipient wetness impregnation procedure. After eachimpregnation, there was a calcination step at about 300° C. to decomposethe nitrate into oxide. This procedure was repeated until the totalamount of active metal was about 40 wt %.

About 5 cc of catalyst was loaded in a micro reactor, diluted in siliconcarbide in a proportion of 6 to 1. The catalyst was reduced at atemperature of about 245° C. for 36 hours, after which the syngas feedwas introduced to the reactor. The reactor run continued without anyinterruptions for catalyst regeneration or rejuvenation for over 4000hours. The results obtained are shown in Table 7 below. The methaneselectivity is lower than 12% and the alpha value is higher than 0.89.

TABLE 7 Performance of a catalyst prepared according to the teachings ofthe present invention hrs on line 800 1200 3000 4000 Pressure, psig 405405 405 405 Temperature, ° F. 375 378 380 384 H2/CO in 1.62 1.62 1.651.65 % CO conversion 50.5 54.3 56.7 60.1 FT rate of reaction 465 467 413410 ccCO conv/cc cat/h CH4 selectivity (C wt %) 7.9 7.2 6.7 6.6 CO2selectivity (C wt %) 0.65 0.64 0.68 0.74 Alpha value 0.92 0.93 0.93 0.93

Whereas, the present invention has been described in relation to thedrawings attached hereto, it should be understood that other and furthermodifications, apart from those shown or suggested herein, may be madewithin the spirit and scope of this invention.

What is claimed is:
 1. A cobalt containing catalyst supported on a metaloxide support, useful for performing the Fischer-Tropsch reaction in afixed bed reactor, which catalyst comprises: a. a metal oxide support ofa pore volume, before loading of cobalt thereon, within the range of0.35 to 0.85 cc/g; b. the metal oxide support having an average porediameter, before the cobalt loading and reduction, as defined by thefollowing equation:Support APD=(0.4372×wt % Co+10.48)×(Area−32)^((0.0113−0.0032×wt % Co))such that the effective average pore diameter, after cobalt loading andreduction is 14 nm or higher; c. a catalyst particle size between 0.4 to1 mm; d. a mechanical strength of the catalyst, if in extrudate form, ofat least 1 lb/mm; e. a methane selectivity below 12 (Carbon) wt %; f. acobalt loading of 11 wt % or higher; g. an alpha value higher than 0.89;and h. a diesel to wax weight ratio below 1.07.
 2. The catalyst of claim1 wherein the catalyst further comprises one or more promoters chosenfrom the group consisting of: Pt, Pd, Ru, Re, Cu and Ag in amountsranging from 0.01 to 3 wt %.
 3. The catalyst of claim 1 wherein thecatalyst further comprises one or more promoters chosen from the groupconsisting of: Mn, Ba, Si, Ti, Ce, Zr, La, Na, K, Zn, Ag, Cu, Li, Ca,Cr, Ni, Fe, V, Sn, Ga, Sb, Th and Win amounts ranging from 0.1 to 15 wt%.
 4. The catalyst of claim 1 wherein the metal oxide is chosen from thegroup consisting of: silica, alumina, titania, zirconia or combinationsthereof.
 5. The catalyst of claim 4 wherein the metal oxide is alumina.6. The catalyst of claim 1 wherein the support is treated with a strongacid before or after extrusion to increase crush strength.
 7. Thecatalyst of claim 6 wherein the strong acid is acetic acid or nitricacid.
 8. A cobalt containing catalyst supported on a metal oxide, usefulfor performing the Fischer-Tropsch reaction in a slurry bubble columnreactor, which catalyst comprises: a. a metal oxide support having apore volume, before loading of cobalt thereon, within the range of 0.35to 0.85 cc/g; b. the metal oxide support having an average porediameter, before the cobalt loading and reduction, as defined by thefollowing equation:Support APD=(0.4372×wt % Co+10.48)×(Area−32)^((0.0113−0.0032×wt % Co))such that the effective average pore diameter, after cobalt loading andreduction, is 14 nm or higher; c. a catalyst particle size between 20 to250 μm; d. a methane selectivity below 12 (Carbon) wt %; e. a cobaltloading of 11 wt % or higher; f. an alpha value higher than 0.89; and g.a diesel to wax weight ratio below 1.07.
 9. The catalyst of claim 8wherein the catalyst further comprises one or more promoters chosen fromthe group consisting of: Pt, Pd, Ru, Re, Cu and Ag in amounts rangingfrom 0.01 to 3 wt %.
 10. The catalyst of claim 8 wherein the catalystfurther comprises one or more promoters chosen from the group consistingof: Mn, Ba, Si, Ti, Ce, Zr, La, Na, K, Zn, Ag, Cu, Li, Ca, Cr, Ni, Fe,V, Sn, Ga, Sb, Th and Win amounts ranging from 0.1 to 15 wt %.
 11. Thecatalyst of claim 8 wherein the metal oxide is chosen from the groupconsisting of: silica, alumina, titania, zirconia or combinationsthereof.
 12. The catalyst of claim 11 wherein the metal oxide isalumina.
 13. The catalyst of claim 8 wherein the support is treated witha strong acid before metal addition to increase crush strength.
 14. Thecatalyst of claim 13 wherein the strong acid includes acetic acid ornitric acid.